1. Field of the Invention
The present invention relates to electrical and electronic components. More specifically, the present invention relates to environmental and electromagnetic seals.
2. Description of the Related Art
Turret assemblies hang on/under multiple airborne and ground based operational platforms including, but not limited to, helicopters, unmanned aerial vehicles, fixed wing aircraft, ground and marine vehicles. As the sophistication of these turret assemblies develops with advances in technology and war-fighting capabilities, electromagnetic interference issues are continuously on the rise.
Electromagnetic interference (EMI) is electromagnetic radiation emitted by electrical circuits carrying rapidly changing signals, as a by-product of their normal operation, which causes unwanted signals (interference or noise) to be induced in other circuits. This interrupts, obstructs, or otherwise degrades or limits the effective performance of those other circuits. EMI can be induced intentionally, as in some forms of external electronic warfare or unintentionally as a result of spurious emissions and the like. EMI emissions can cause severe operational platform complications such as flight, guidance, and weapon control interference. In addition, any system emitting large EMI emissions can easily be detected, and tracked by enemy forces.
Many current turret systems are complex in design and contain numerous electrical components and assemblies. In turret systems, gimbals are typically the heart of rotational motion. These gimbals consist of two dynamic rotating components (azimuth and elevation) to accurately distinguish, track, and eliminate targets. The dynamic azimuth and elevation interfaces are bearing supported, motor driven joints that contain dynamic environmental seals. The motors are often powered with high frequency voltages that can generate extremely strong EMI emissions.
One method to suppress internally generated or externally induced EMI is a Faraday cage. A Faraday cage is an enclosure designed to exclude electromagnetic fields in an application of one of Maxwell's equations: Gauss's law. Gauss's law describes the distribution of electrical charge on an electrically conductive form, such as a sphere, a plane, a torus, a gimbal etc. Since like charges repel each other, charge will ‘migrate’ to the surface of the conducting form. By adding a network of conductive contacts between rotating gimbal interfaces, a Faraday cage volume can be created. As internally generated EMI approaches the inside wall of the gimbal Faraday cage volume, charge will be absorbed and transferred back to chassis ground. This absorption eliminates EMI emissions from exiting the gimbal.
Given the inherent rotational characteristics of gimbals, completing a Faraday cage volume can be difficult and expensive to accomplish. Ball bearings enable gimbals to rotate and they are generally made from low electrically conductive steel. In addition, lubricated single point bearing contacts between the inner and outer races have extremely high electrical resistance values. Electrical resistance “R”, measured in Ohms Ω, is a physical material property that impedes electrical current flow. In other words, a material with a high electrical resistance value is considered a poor conductor of electricity. Conversely, a material with a low electrical resistance value is a good conductor of electricity.
In many turret systems, maximum electrical resistance values are strictly enforced by customer specifications. These values are measured from the inner turret sensor, through the gimbal (including bearings and seals), and eventually to chassis ground. In past turret systems, 1Ω to 4Ω was considered to be an acceptable maximum resistance value. Today less than 2.5 milli-ohms is required to meet customer/program requirements. Such a change is a 1600 fold decrease in electrical resistance.
With increasingly complicated electronics, stronger torque motors, varying materials, and the general rotational dynamics of these sophisticated turrets, EMI becomes more difficult to contain and eliminate. In addition, as enemy countermeasures continue to develop, it is necessary to suppress these radiated EMI emissions even further than before.
Hence, there is a need in the art for an improved system or method for shielding systems from electromagnetic interference.